The present invention generally relates to the field of electronic assembly and more particularly relates to electronic component interconnects in electronic assemblies.
Electronic assemblies typically employ printed circuit boards for mechanical support and electrical interconnection of electronic components. In the case of surface mount electronic components, component leads are placed onto metal pads deposited onto a surface of the boards. Solder is used to attach the components leads to the pads and to electrically connect the components leads to the pads. The pads are electrically interconnected by traces, or connected to an internal conductive plane of the circuit board through a metal plated aperture extending into the circuit board, called a via, so as to provide for electrical interconnection of the components.
In an assembly process, solder paste is placed onto the pads on a first side of the printed circuit board. Components are arranged on the first side of the circuit board so that component leads cling to the solder paste in appropriate locations. The paste is heated to solder the components to the pads on the first side of the printed circuit board. The solder is allowed to cool and harden.
After the solder on the first side of the printed circuit board is hardened, enhanced component density is achieved by turning the printed circuit board over, placing solder paste onto the pads on the other side of the printed circuit board and arranging components on the other side of the circuit board so that component leads cling to the solder paste in appropriate locations. The paste is heated to solder the components to the pads on the other side of the printed circuit board. The solder is again allowed to cool and harden.
It should be understood that flexibility in order of soldering operations is limited since the solder paste on the first side of the circuit board is heated, cooled and hardened before any solder paste is applied to the other side of the printed circuit board. If solder paste and components were applied to both sides of the printed circuit board before any heat were applied to either side, it is likely that the pull of gravity or vibrations would displace some of the components, since the solder paste has insufficient adhesive power to retain positioning of components so effected. Flexibility in order of soldering operations is particularly limited when the components include components that must be soldered to pads on both sides of the printed circuit board, such as straddle mount connectors.
It is important to achieve and maintain correct positioning of components in soldering operations. Misregistered component leads cause electrically or mechanically defective solder joints, requiring costly and time consuming rework operations. Misregistered component leads may also cause circuit shorting and catastrophic electrical failure of the entire electronic assembly.
What is needed is a method and apparatus for printed circuit board pads with registering feature for component leads.
The present invention provides a method and apparatus for printed circuit board pads with registering feature for component leads.
Briefly, and in general terms the invention provides a U-shaped metalized pad on a printed circuit board for soldering to a component lead. Solder is disposed on the pad and heated to a molten state so that surface tension and wetting effects form the molten solder into a solder mound having a U-shaped lateral cross section conforming to the U-shaped metalized pad. The solder mound has a first arm and a second arm, and a lateral aperture extending therebetween for receiving an extremity of the component lead. The solder mound also has a bridge portion extending between the first and second arms of the solder mound. The solder mound is cooled so as to harden the solder mound.
The extremity of the component lead is disposed proximate to the lateral aperture between the first arm and second arm of the solder mound. By sliding the extremity of the component lead in contact with the first arm and second arm of the solder mound, the component lead is registered with respect to the pad in a lateral dimension perpendicular to the arms of the mound. The lateral aperture of the solder mound receives the extremity of the component lead. By sliding the extremity of the component lead into contact with the bridge portion of the solder mound, the component lead is registered with respect to the pad in a lateral dimension perpendicular to the bridge portion of the solder mound. The solder mound is then heated to a molten state so as to solder the component lead to the pad.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.